You can only connect the school to an admin account after the School Administrator has created a ClassTag Leader account. Simply invite the school administrator to join as a ClassTag Leader by following the instructions in this article.
Once added as a ClassTag School Leader, the school admin personnel can manage the classes listed for the school on ClassTag and invite his/her class and other teachers’ classes on the school’s dashboard. The administrator can either click on Assign to assign their class to the school or click on Invite an Existing Classroom to invite other teachers’ classrooms.